A variety of types of chemical sensors have been used in the detection of various chemical processes. One type is a chemically-sensitive field effect transistor (chemFET). A chemFET includes a source and a drain separated by a channel region, and a chemically sensitive area coupled to the channel region. The operation of the chemFET is based on the modulation of channel conductance, caused by changes in charge at the sensitive area due to a chemical reaction occurring nearby. The modulation of the channel conductance changes the threshold voltage of the chemFET, which can be measured to detect and/or determine characteristics of the chemical reaction. The threshold voltage may for example be measured by applying appropriate bias voltages to the source and drain, and measuring a resulting current flowing through the chemFET. As another example, the threshold voltage may be measured by driving a known current through the chemFET, and measuring a resulting voltage at the source or drain.
An ion-sensitive field effect transistor (ISFET) is a type of chemFET that includes an ion-sensitive layer at the sensitive area. The presence of ions in an analyte solution alters the surface potential at the interface between the ion-sensitive layer and the analyte solution, usually due to the dissociation of oxide groups by the ions in the analyte solution. The change in surface potential at the sensitive area of the ISFET affects the threshold voltage of the device, which can be measured to indicate the presence and/or concentration of ions within the solution. Arrays of ISFETs may be used for monitoring chemical reactions, such as DNA sequencing reactions, based on the detection of ions present, generated, or used during the reactions. See, for example, U.S. Pat. No. 7,948,015 to Rothberg et al., which is incorporated by reference herein in its entirety. More generally, large arrays of chemFETs or other types of chemical sensors may be employed to detect and measure static and/or dynamic amounts or concentrations of a variety of analytes (e.g. hydrogen ions, other ions, compounds, etc.) in a variety of processes. The processes may for example be biological or chemical reactions, cell or tissue cultures or monitoring, neural activity, nucleic acid sequencing, etc.
As sensor technology improves, the ability to measure or detect minute changes within an environment or low concentrations of chemical species also improves. Such improvement is particularly true for chemical and biological sensors, such as sensors for detecting the presence of chemical species, particularly those relevant to molecular biology, or for genetic genotyping or sequencing. With the effort to detect ever smaller changes or ever lower concentrations, noise within circuitry associated with sensors becomes an increasing problem. Moreover, as sensors become integrated with processing or memory devices, noise within the system can cause increasingly large propagating errors. Such errors can lead to missed data, mischaracterized data, or combination thereof. As such, an improved system would be desirable.